Technology update

Oct 4, 2013

Adding holes to graphene

Graphene (a sheet of carbon just one atom thick) can be successfully p-doped with boron, according to new experiments by researchers at Columbia University in the US. The result means that the carbon material can be both electron- and hole-doped, something that is important for advancing real-world graphene-based electronics.

Chemical doping can be used to add or remove electrons from a material. Two years ago, a team led by Abhay Pasupathy showed that graphene could be n-doped using nitrogen atoms. Now, the same team has proved that the carbon material might also be doped with boron – which takes away electrons from graphene instead of adding them. Such complementary doping is a key feature behind all silicon transistor technology and the new result shows that these processes can be applied to graphene too.

In n-doping, the extra nitrogen atoms do not significantly modify the basic structure of graphene sheets and the Columbia researchers have now found that the same goes for boron too. Silicon itself behaves similarly: a small amount of phosphorus is routinely used to dope the material but it does not fundamentally change its basic structure either.

Half a hole into the graphene sheet

In the new work, Pasupathy and colleagues studied graphene films that they had grown by passing carbon- and boron-containing gases over crystalline copper foil at 1000 °C. The researchers then used scanning tunnelling microscopy and spectroscopy to look at the local structure and electronic properties of single graphitic boron dopants. "Like in nitrogen-doped graphene, we found that one boron atom bonds to three neighbouring carbon atoms in the graphene lattice," Pasupathy told nanotechweb.org. "From the spectroscopic measurements, we learnt that each boron dopant contributes roughly half a hole into the graphene sheet." A hole is the absence of an electron.

The results, published in Nano Lett. DOI: 10.1021/nl401781d, provide new insights into the electronic effects of doping graphene by chemical means, he added, which is essential for graphene-based electronic applications. "Doping graphene in this way could allow us to create transparent electrodes, and the dopants themselves might also act as reactive centres – something that would allow us to further functionalize graphene for sensor and related applications."

The team, which includes scientists from Cornell University, SLAC National Accelerator Laboratory, the National Institute of Standards and Technology and Brookhaven National Laboratory, is now busy looking at how to make p-n junctions from graphene samples doped with both nitrogen and boron. "The p-n junction or diode is the fundamental building block of electronic circuitry," explains Pasupathy, "and we are attempting to build one in graphene without relying on electrostatic gates."

Graphene is a sheet of carbon atoms arranged in a honeycomb-like lattice just one atom thick. Often called the "wonder material", it has been touted by some to replace silicon as the electronic industry's material of choice in the future thanks to its many unique electronic and mechanical properties.